A single in-line memory module (SIMM) comprises a circuit panel card having a plurality of integrated circuit components mounted thereon, with circuit traces extending from the components to contact pads along an edge of the card. A SIMM card has redundant sets of circuit traces and contact pads on its opposite sides, while a dual in-line memory module (DIMM) card has independent sets of circuit traces and contact pads on each of its opposite sides. Both SIMM cards and DIMM cards can be mounted as daughtercards on a circuitboard mothercard.
Sockets are well-known to receive the edge of a circuit panel daughtercard, either SIMM or DIMM, and establish an electrical interconnection with a circuitboard mothercard. These sockets have a slot for receiving the edge of the daughtercard, and contacts which extend into the slot to engage the contact pads along the edge of the daughtercard. During insertion of the daughtercard into the slot, the contacts are resiliently deflected within their individual cavities, thereby generating a normal force on the contact pads which is necessary to provide a good electrical interconnection. However, the required deflection of the contacts also generates a resistance to insertion of the card, and as the required normal force and/or the number of contacts in the socket increases, the resistance to insertion becomes quite large.
In order to alleviate the problem of insertion resistance, zero insertion force sockets have been developed. These sockets, known as pivot-in or cam-in sockets, permit a daughtercard to be inserted into the socket in an initial orientation with a zero insertion force, and then to be pivoted to a final orientation during which the contacts are more easily deflected due to a camming action of the daughtercard. A drawback of the zero insertion force sockets is that additional space is required surrounding the socket in order to permit the pivoting action of the daughtercard. In electronic devices where space is at a premium, it is often desirable to utilize straight-in, or direct insert, sockets.
The design of direct insert sockets and contacts generally involves a compromise between the opposed objectives of providing a high normal force on the contact pads while providing a low resistance to insertion of the card. A known device for reducing the insertion resistance while maintaining a desired normal force is to stagger the contacts at different heights in the socket, as disclosed in U.S. Pat. No. 5,112,231. This device is effective because insertion resistance has two components, namely deflection resistance due to spring deflection of the contacts, and friction resistance due to sliding over the contacts as the card is being inserted. The deflection resistance is generally the greater of the two, By separating the deflection of the contacts into two or more stages, the maximum insertion resistance is reduced by a significant amount. However, this device requires that the daughtercard have contact pads at two distinct levels, and results in the socket and the card having a greater height than is the case for a card which has contact pads at one level only.
The present invention provides a socket with a low insertion force contact system which reduces the insertion resistance while permitting acceptance of a daughtercard having contacts pads all at one level, thereby enabling the socket to have a low overall height.